Loss of organs or appendages can result from congenital defects, injury or disease. Many times treatment with drugs or surgery is not in itself sufficient and the patient is severely disabled. For example, burn victim are often disfigured by the loss of cartilage in appendages such as the nose, ears, fingers or toes. One approach for treatment has been to transplant donor organs or tissue into the patient, or graft tissues from one area of the patient to another. However, there is a tremendous shortage of donor organs, most of which must come from a recently deceased individual. Moreover, these tissues have generally not been useful for correction of defects such as worn or torn cartilage.
Cell transplantation has been explored as an alternative to various means of replacing tissue function. Using this approach, individual cells are harvested from a healthy section of donor tissue, isolated and implanted in the patient at a desired site. Cell transplantation has several advantages over whole organ transplantation. Because the isolated cell population can be expanded in vitro using cell culture techniques, only a very small number of donor cells are needed to prepare an implant. Consequently, the living donor need not sacrifice an entire organ. The use of isolated cells also allows removal of other cell types which may be the target of immune responses, thus diminishing the rejection process. In addition, major surgery on the recipient and donor and its inherent risks are avoided. Finally, the cost of the procedure may be significantly reduced.
There have been a number of attempts to culture dissociated tissue and implant the cultured cells directly into the body. For example, transplantation of pancreatic tissue, either as a whole organ or as a segment of an organ, into the diabetic patient has been attempted. However, such implants do not form three dimensional structures, and the cells are lost by phagocytosis and attrition.
Isolated cells cannot form new tissues on their own. Most cells have a requirement for attachment to a surface in order to replicate and function. They require specific environments which very often include the presence of supporting material to act as a template for growth. Three dimensional scaffolds are used to mimic their natural counterparts, the extracellular matrices of the body. They serve as both a physical support and an adhesive substrate for isolated parenchymal cells during .uparw.in vitro.uparw. culture and subsequent implantation.
A method for forming artificial skin by seeding a fibrous lattice with epidermal cells is described in U.S. Pat. No. 4,485,097 to Bell, which discloses a hydrated collagen lattice that, in combination with contractile agents such as platelets and fibroblasts and cells such as keratinocytes, is used to produce a skin-like substance. U.S. Pat. No. 4,060,081 to Yannas et al. discloses a multilayer membrane for use as a synthetic skin that is formed from an insoluble modified collagen material that is very slowly degradable in the presence of body fluids and enzymes. U.S. Pat. No. 4,458,678 to Yannas et al. discloses a process for making a skin-like material wherein a biodegradable fibrous lattice formed from collagen cross-linked with glycosaminoglycan is seeded with epidermal cells. Unfortunately, there is a lack of control over the composition and configuration of the latter matrix because it is primarily based on collagen. In addition, the degradation is quite variable because the collagen is degraded by enzymatic action and hydrolysis.
U.S. Pat. No. 4,520,821 to Schmidt describes a similar approach to make linings to repair defects in the urinary tract. Epithelial cells are implanted onto the surface a liquid impermeable synthetic polymeric matrix where they form a monolayer lining on the matrix. This approach is limited to the production of relatively thin structures.
Vacanti, et al., "Selective cell transplantation using bioabsorbable artificial polymers as matrices" J. Pediat. Surg. 23, 3-9 (1988) and Vacanti, "Beyond Transplantation" Arch. Surg. 123, 545-549 (1988), describe an approach for making new organs for transplantation which was not subject to the same limitations as the work of Yannas and Burke, i.e., it was not limited to the construction of very thin organs such as skin. Vacanti, et al., recognized that cells require a matrix for attachment and support if they are to survive following implantation, that a minimum number of cells was essential for function in vivo, and that the matrix must be porous enough to allow nutrients and gases to reach all of the cells on and within the matrix by diffusion, until the matrix-cell structure was vascularized. Moreover, they recognized the advantage of using synthetic biodegradable polymer substrates to form a scaffold that mimics its natural counterparts, the extracellular matrices (ECM) of the body, serving as both a physical support and an adhesive substrate for isolated parenchymal cells during in vitro culture, and subsequent implantation, degrading as the cells begin to secrete they own ECM support. Subsequent studies have demonstrated that even better results are obtained when the matrix is first implanted, prevascularized, and then seeded with cells. Most matrices used in the earlier work are modifications of materials already available, such as surgical sutures and meshes. This latter approach, however, requires new matrix configurations which are optimal for vascularization, yet resistant to compression, with sufficient porosity and interconnected interstitial spacings to allow injected cells to become dispersed throughout the matrix.
As a result, there remains a need for improved polymeric matrices that provide guided support for the cells to be implanted, especially in the reconstruction of structural tissues like cartilage and bone, where tissue shape is integral to function. In particular, there is a need for a matrix that is relatively easy to manufacture in a shape appropriate for a particular patient, which can be constructed to degrade in synchrony with the growth of the cells seeded thereon. A uniformly distributed and interconnected pore structure is important so that an organized network of tissue constituents can be formed. Therefore, these scaffolds must be processable into devices of varying thickness and shape.
To date, no processing techniques exist to prepare three-dimensional biocompatible foams with complex and delicate shapes, thereby limiting the potential of organ regeneration by cell transplantation.
It is therefore an object of the present invention to provide a method of preparing highly porous, biocompatible polymer membranes, and the resulting membranes.
It is a further object of the present invention to provide a highly porous, biocompatible three-dimensional matrix having a desired anatomical shape.
It is another object of the present invention to provide a three-dimensional biocompatible matrix for reconstruction of anatomically-shaped tissues, especially cartilage and bone.